Oxidative addition of palladium(0) complexes generated from

The major complex formed in solution from [[Pd0(dba)2]+1P-N] mixtures is [Pd0(dba)(P-N)] (dba=trans,trans-dibenzylideneacetone; P-N=PhPN, 1-dimethylamino-2-diphenylphosphinobenzene; FcPN, N,N-dimethyl-1-[2-(diphenylphosphino)ferrocenyl]methylamine; OxaPN, 4,4'-dimethyl-2-(2-diphenylphosphinophenyl)-1,3-oxazoline). Each complex consists of a mixture of isomers involved in equilibria: two 16-electron rotamer complexes [Pd0(eta2-dba)(eta2-P-N)] and one 14-electron complex [Pd0(eta2-dba)(eta1-P-N)] observed for FcPN and OxaPN. [Pd0(dba)(PhPN)] and [SPd0(PhPN)] (S solvent) react with PhI in an oxidative addition: [SPd0(PhPN)] is intrinsically more reactive than [Pd0(dba)(PhPN)]. This behavior is similar to that of the bidentate bis-phosphane ligands. When the PhPN ligand is present in excess, it behaves as a monodentate phosphane ligand, since [Pd0(eta2-dba)(eta1-PhPN)2] is formed first by preferential cleavage of the Pd-N bond instead of the Pd olefin bond. [Pd0(eta1-PhPN)3] is also eventually formed. [Pd0(dba)(FcPN)] and [Pd0(dba)(OxaPN)] are formed whatever the excess of ligand used. [SPd0(FcPN)] and [SPd0)(OxaPN)] are not involved in the oxidative addition. The 16-electron complexes [Pd0(eta2-dba)(eta2-FcPN)] and [Pd0(eta2-dba)(eta2-OxaPN)] are found to react with PhI via a 14-electron complex as has been established for [Pd0(eta2-dba)(eta1-OxaPN)]. Once again, the cleavage of the Pd-N bond is favored over that of Pd-olefin bond. This work demonstrates the higher affinity for [Pd0(P-N)] of dba compared with the P-N ligand, and emphasizes once more the important role of dba, which either controls the concentration of the most reactive complex, [SPd0(PhPN)], or is present in the reactive complexes, [Pd0(dba)(FcPN)] or [Pd0(dba)(OxaPN)], and thus contributes to their intrinsic reactivity.     

Phosphorus-nitrogen-phosphorus ligands: cooperative effects between nitrogen and phosphorus substituents on catalytic activity

A new generation of PNP compounds bearing different diarylphosphine groups were prepared and used as ligands in palladium-catalysed Suzuki cross-coupling reactions. Rates of oxidative addition of iodobenzene to (PNP)Pd[0] complexes were measured using UV spectroscopy. Synergistic effects between the N- and P- substituents were identified and correlated in redox and catalytic chemistry.     

Active anionic zero-valent palladium catalysts: characterization by density functional calculations

This works uses DFT (B3LYP/LACVP*(+)//B3LYP/LACVP* level) to ascertain the existence of the tricoordinate, anionic zero-valent palladium complexes that were postulated as the active species in the catalytic cycles of Pd-catalyzed Heck and cross-coupling reactions. The variety of complexes studied (1 and 2), include [Pd(PR(3))(2)X](-) species, in which R=H, Me, vinyl, and phenyl, and X=Cl, Br, I, AcO, and TFA, as well as bidentate complexes, [Pd[Ph(2)P(CH(2))(n)Ph(2)P]X](-), in which X=Cl, AcO and n=3-6. The study shows that these complexes exist as distinct minima in the gas phase as well as in THF. In addition, it provides geometric features and Pd--X(-) dissociation energies for all these complexes as well as some NMR and IR data, which show a clear distinction in these features between the tri- and dicoordinate Pd(0) species. An orbital interaction model and perturbation theory arguments account for the bonding mechanism and rationalize all the trends in the stability of the Pd--X bond. These trends include the effects of variation of X, R, and the length of the linker in the bidentate ligands.     

Efficient palladium-catalyzed synthesis of unsymmetrical donor—acceptor biaryls and polyaryls

4,4′-Unsymmetrically substituted biphenyls can be synthesized by cross-coupling reactions of substituted aromatic organometallic reagents and aromatic halides catalyzed by palladium complexes. This two-step method from commercially available aromatic halides has been used for the synthesis of a series of donor/acceptor para-substituted biphenyls, D---C₆H₄---C₆H₄---A, where D is an electron donor group and A an electron acceptor group, which are of interest as liquid crystal precursors and as having potential in non-linear optics. Biaryls in which the donor-phenyl moiety is replaced by a 2-furyl or 2-thienyl can be synthesized similarly. The method can also be used for the convergent synthesis of previously unreported unsymmetrically substituted polyparaphenylenes (n = 3,4).     

Immobilization and Stabilization of Xylanase by Multipoint Covalent Attachment on Agarose and on Chitosan Supports

Xylanases have important applications in industry. Immobilization and stabilization of enzymes may allow their reuse in many cycles of the reaction, decreasing the process costs. This work proposes the use of a rational approach to obtain immobilized commercial xylanase biocatalysts with optimized features. Xylanase NS50014 from Novozymes was characterized and immobilized on glyoxyl-agarose, agarose-glutaraldehyde, and agarose-amino-epoxy support and on differently activated chitosan supports: glutaraldehyde-chitosan, glyoxyl-chitosan, and epoxy-chitosan. Two different chitosan matrices were tested. The best chitosan derivative was epoxy-chitosan-xylanase, which presented 100% of immobilization yield and 64% of recovered activity. No significant increase on the thermal stability was observed for all the chitosan-enzyme derivatives. Immobilization on glyoxyl-agarose showed low yield immobilization and stabilization degrees of the obtained derivative. The low concentration of lysine groups in the enzyme molecule could explain these poor results. The protein was then chemically modified with ethylenediamine and immobilized on glyoxyl-agarose. The new enzyme derivatives were 40-fold more stable than the soluble, aminated, and dialyzed enzyme (70 °C, pH 7), with 100% of immobilization yield. Therefore, the increase of the number of amine groups in the enzyme surface was confirmed to be a good strategy to improve the properties of immobilized xylanase.     

Formation of anionic palladium(0) complexes ligated by the trifluoroacetate ion and their reactivity in oxidative addition

As established previously for Pd(OAc)₂, Pd⁰ complexes are formed in situ from Pd(OCOCF₃)₂ and n equiv. triarylphosphines (4-Z-C₆H₄)₃P (Z = CF₃, F, Cl, H, CH₃; n ? 3). The phosphines are the intramolecular reducing agents and are oxidized to triarylphosphine oxides. The generated Pd⁰ complexes are anionic species ligated by the trifluoroacetate anion: Pd⁰(PAr₃)_n(OCOCF₃)⁻ (n = 2 or 3). Pd⁰(PAr₃)₂(OCOCF₃)⁻ is the reactive species involved in the oxidative addition to PhI. This leads to trans-PhPd(OCOCF₃)(PPh₃)₂, involved in equilibrium with the cationic complex trans-[PhPd(PPh₃)₂(DMF)]⁺, instead of the expected trans-PhPdI(PPh₃)₂ complex. The existence of anionic Pd⁰ complexes ligated by the acetate or trifluoroacetate ions delivered by the precursors Pd(OAc)₂ or Pd(OCOCF₃)₂, respectively, as well as their comparative reactivity in oxidative additions are consistent with theoretical DFT calculations.     

Autocatalysis for C-H bond activation by ruthenium(II) complexes in catalytic arylation of functional arenes

Kinetic data for the C-H bond activation of 2-phenylpyridine by Ru(II)(carboxylate)(2)(p-cymene) I (acetate) and I' (pivalate) are available for the first time. They reveal an irreversible autocatalytic process catalyzed by the coproduct HOAc or HOPiv (acetonitrile, 27 °C). The overall reaction is indeed accelerated by the carboxylic acid coproduct and water. It is retarded by a base, in agreement with an autocatalytic process induced by HOAc or HOPiv that favors the dissociation of one carboxylate ligand from I and I' and consequently the ensuing complexation of 2-phenylpyridine (2-PhPy). The C-H bond activation initially delivers Ru(O(2)CR)(o-C(6)H(4)-Py)(p-cymene) A or A', containing one carboxylate ligand (OAc or OPiv, respectively). The overall reaction is accelerated by added acetates. Consequently, C-H bond activation (faster for acetate I than for pivalate I') proceeds via an intermolecular deprotonation of the C-H bond of the ligated 2-PhPy by the acetate or pivalate anion released from I or I', respectively. The 18e complexes A and A' easily dissociate, by displacement of the carboxylate by the solvent (also favored by the carboxylic acid), to give the same cationic complex B(+) {[Ru(o-C(6)H(4)-Py)(p-cymene)(MeCN)](+)}. Complex B(+) is reactive toward oxidative addition of phenyl iodide, leading to the diphenylated 2-pyridylbenzene.     

Fluorophores related to the green fluorescent protein

Imidazolin-5-one derivatives and isosteres (oxazolinones, butenolides, and pyrrolinones) of the 4-hydroxybenzylidene-imidazolinone chromophore of the GFP have been synthesized and their photophysical properties have been investigated.